Slippery when windy

If you live in a city it might not occur to you that wind is a big design consideration. In a congested environment the wind can be turbulent and nasty at times, but rarely is it an actual threat to a building unless it is a tornado or hurricane type wind event. If you live on the front range of the Rocky Mountains you can add the Chinook winds that often make winter interesting times.

Wind is a deceptive force. When it is flowing at low speed it cools us and makes life bearable. The faster it flows, the less comfortable it gets until it causes massive destruction. This is because the force of the wind on any object in it’s path increases exponentially to it’s speed. This is to say that if the wind speed doubles, the force it exerts is squared. While this makes it a great energy resource at lower speeds, it is also a recipe for disaster.

Wind generators and water pumping windmills must be designed to withstand huge stresses as it’s impossible to take them in every time a strong weather front passes. I have lost several wind generators to fronts that supplied several days of sustained 40+mph wind. It is a personal challenge that keeps many busy for years, building a system that doesn’t self destruct in strong wind. Unfortunately, we don’t have the luxury of building home after home in the same way. It’s important to design for the worst possible wind event in order for a home to survive, especially in an open environment where it is exposed directly to the wind. Where I live there are no trees, hills or other impediments to wind flow for miles. My house is in the same position a wind generator would be, totally exposed.

Having experienced the power of the wind growing up in Boulder, Colorado, where 80+ mph wind was common in January and February, and seeing homes with windows blown out and roofs blown off, this was at the forefront of my mind as I designed my home. My inclination to build a round home developed into a determination as I studied the effect of wind on various building shapes. The fact is that flat straight walls are a barrier to wind flow, where curved walls allow wind to slip around them. The wind load on curved walls is reduced while at the same time the walls are less likely to collapse due to their shape. It’s not all that simple though as the wind provides multiple types of force on a building including uplift on the roof and a pulling on the downwind walls. While a round shape will reduce several of these forces, it does not eliminate them.

While anecdotal, I believe my experience with the round house on the prairie can be instructive. The house is a 40ft diameter circle. From the north, where the prevailing wind comes from, the roof looks most like an African hut or a squat tipi shape. The only dormer faces South. During strong winds the air flows around the house. While there will always be some pressure build up on the windward side of any structure, there is no impediment to flow around a round building. During blizzards this is especially visible as the snow flows with the stronger wind and settles out of the slower wind.

The winter of 2006-2007 was a very snowy and windy time here. Every weekend for 8 weeks we had blizzards that made keeping the driveway open a never ending job. A snow fence blocking the wind from the trailer we were living in while building our home had over six feet of snow behind it before it began to thaw. The photo below shows about 3 feet of that drift.

Snow piles up behind a snow fence

Around the house though, the ground was bare of snow while 10 feet from the house the drifts were 3 feet deep. It was 4 feet deep in parts of the driveway. The photo below is from an early storm when the drifts were not so deep. To the right of the picture is a tall spire of snow reaching to the second story. This spire was left by a vortex that was created on the downwind side of the house. In dryer winds this is where tumbleweeds are deposited.

Wind flow around the round shape scours a clear path

A few quick references I found:

http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd068_e.html – an article from 1965, but covers the basics.